The aim of this research is to maximize fluorapatite formation in enamel from topical application of aqueous fluoride solutions. In the past year, we studied in vitro the effect of solutions of sodium, potassium and ammonium fluoride with or without added orthophosphate at neutral and acidic pH. Fluoride from ammonium fluoride penetrated the enamel most readily while the presence of phosphate tended to decrease fluoride penetration, presumably through depressing enamel dissolution during the application. Orthophosphate tended to enhance fluorapatite formation. Most fluorapatite resulted from prolonged application of acidic combinations of ammonium fluoride and phosphate. It was further found that the beneficial effects on fluorapatite formation from a post-treatment incubation could be mimicked by covering the enamel with a lacquer and storing it under water or saliva. Work for the coming year is aimed at determining which combinations of phosphate and fluoride are optimal for fluorapatite formation in intact enamel. Other experiments will be concerned with surface active agents and their influence on the speed of fluoride interactions in depth as well as on the quantity and the quality of the reaction products.